Marine seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) under the seafloor. This profile does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. In other words, such an image of the subsurface is a necessary tool today for those drilling exploration wells for minimizing the potential of finding a dry well. Thus, providing a better image of the subsurface is an ongoing process.
For a traditional, narrow azimuth seismic gathering process, as shown in FIG. 1, a marine seismic data acquisition system 100 includes a survey vessel 102 towing a plurality of streamers 104 (one shown) that may extend over kilometers behind the vessel. One or more source arrays 106 (or simply “source”) may also be towed by the survey vessel 102 or another survey vessel (not shown) for generating seismic waves 108. Conventionally, the source arrays 106 are placed in front of the streamers 104, considering a traveling direction of the survey vessel 102. The seismic waves 108 generated by the source arrays 106 propagate downward and penetrate the seafloor 110, eventually being reflected by a reflecting structure 112, 114, 116, 118 at an interface between different layers of the subsurface, back to the surface 119. The reflected seismic waves 120 propagate upward and are detected by detectors 122 provided on the streamers 104. This process is generally referred to as “shooting” a particular seafloor 110 area.
One of the shortcomings of existing technology relates to the poor azimuth/offset distribution of the data collection points, i.e., detectors 122, positioned along streamers of equal length, and the number of streamers 104 attached to the survey vessel 102. Generally, a single survey vessel 102 tows approximately ten to sixteen streamers 104, of uniform length, with detectors 122 equally spaced along the length of each streamer. In this configuration, the azimuth of the collection points is narrow. The azimuth is defined as the angle made between a line that passes through the source and a recording receiver and the navigation path when viewed from above the source and the recording receiver. Narrow azimuth distribution (NAZ), which is typical for a single vessel seismic survey, leads to problems associated with multiple (reflective) removals at locations on the streamers in close proximity to the source arrays 106.
Another shortcoming associated with existing NAZ acquisition methods relates to the collected data in relation to its intended use, i.e., different streamer collection configurations lend themselves to different uses of the data, such as multiple removal, imaging and model building. Narrow azimuth distribution streamer configurations are not focused on a specific use of the collected data, resulting in less than optimal seismic image results.
To achieve a good image of the surveyed subsurface, an ideal set of seismic data will provide complete and uniform illumination of the subsurface. Uniform illumination requires that each point in the subsurface is represented by a fold of data that corresponds to a uniform distribution of source-receiver offsets and a uniform distribution of source-receiver azimuths. As illustrated in FIG. 2A, a typical NAZ system that includes only a vessel 202, a source 206 and a set of streamers 204, is able to collect seismic data having a single azimuth and limited source-receiver offsets. An improvement to the NAZ system is illustrated in FIG. 2B, and it includes a plurality of NAZ systems (only three are shown in the figure). This system is known in the art as multi-azimuth (MAZ) system. This system achieves a better azimuth distribution as the vessel 202 generates, for the same subsurface point, multiple azimuths.
Another improvement of the NAZ system is illustrated in FIG. 2C, and includes two more vessels 202′ and 202″ that tow corresponding sources 206′ and 206″, where these additional vessels advances on lines parallel to the vessel 202's path, thus achieving better source-receiver offset and better azimuth. This system is known in the art as a wide-azimuth WAZ system.
Still another improvement, developed by the assignee of this application, is illustrated in FIG. 2D (which corresponds to FIG. 4 of U.S. patent application Ser. No. 14/902,926, the entire content of which is incorporated by reference herein). This system, called StagSeis, includes two streamer vessels 202 and 204 and three source vessels 206, 208 and 210. The streamer vessels 202 and 204 are towing corresponding streamer spreads 202B and 204B, and optionally, one or more seismic source arrays 202A and 204A. A source array may include one or more sub-arrays and a sub-array may include one or more source elements. A source element may be an air gun, a vibratory element, etc. A streamer vessel necessarily tows a streamer spread while a source vessel necessarily tows a source array. However, it is possible that the streamer vessel also tows a source array, as illustrated in FIG. 2D. In this StagSeis configuration, the sources are staggered both along the inline direction X and the cross-line direction Y. The StagSeis configuration may have the sources distributed along a curved line 220. In one application, line 220 is a straight line, slanted or not relative to the advancing path of the vessels.
Still another configuration used in the industry for improving the azimuth distribution is the Coil configuration, which is illustrated in FIG. 2E. According to this configuration, vessel 202 follows an almost circular path (i.e., a coil).
However, although the above noted systems improve the source-receiver offset and azimuth distributions, there is still a need for collecting data with even better distributions for further improving the accuracy of the image of the surveyed subsurface (which is obtained by processing the acquired seismic data). Accordingly, it would be desirable to provide systems and methods that further improve the azimuth distribution and/or the source-receiver offset distribution for a marine seismic acquisition system to improve an accuracy of the subsurface's image.